The present invention relates to an aberration-correcting optical relay for an optical system.
The invention relates more particularly, but not exclusively, to reflector telescopes.
It is known that the various aberrations that arise in optical systems that include mirrors can be corrected by using mirrors of complex shape, e.g. of parabolic shape, and/or by including in a particular optical system full aperture corrector elements such as Schmidt blade or a Maksutov meniscus when the optical system is arranged as a reflector telescope.
Such correction means have the drawback of being complex to implement, and thus of giving rise to high costs.
The present invention seeks to correct aberrations of geometrical and/or chromatic type in an optical system in a manner that is simpler and less expensive.
When the optical system is arranged as a telescope having a primary mirror and a secondary mirror, the invention also makes it possible to reduce significantly the diameter of the secondary mirror.
The invention also makes it possible to transfer the image of the optical system from a position that is of difficult access or inaccessible to a position that is accessible for observation.
The invention achieves this by means of an aberration-correcting optical relay comprising two converging optical units, a front unit and a rear unit, and a correcting meniscus or two correcting meniscuses placed symmetrically relative to each other, the meniscus(es) having substantially concentric main faces, the two converging optical units being placed on the same axis and the correcting meniscus(es) being placed on said axis between the two converging optical units, the front converging optical unit situated upstream from the correcting meniscus(es) being placed in such a manner that the distance from an image point of the portion of the optical system upstream of the optical relay to the front converging optical unit is equal to the focal length of the front converging optical unit, said unit thus transforming a beam coming from said image point into a parallel beam.
When the image point is on the axis of the optical relay, the above-mentioned parallel beam strikes the meniscus parallel to the axis.
When the image point is off-axis, the above-mentioned parallel beam strikes the meniscus in a manner that is inclined relative to the axis.
The meniscus(es) are preferably provided with main faces that are exactly concentric.
The term xe2x80x9ctwo correcting meniscuses placed symmetrically relative to each otherxe2x80x9d is used to mean two meniscuses that are placed in such a manner that their centers of curvature coincide.
The meniscus(es) can also be of small diameter and easy to make, thus enabling the optical relay to be advantageous in terms of cost.
By means of the invention, aberrations can be corrected solely at the aberration-correcting optical relay.
The invention thus makes it possible to eliminate the use of mirrors that are of complex shape and of full aperture corrector elements in the optical system.
At the outlet from the correcting meniscus, the beam becomes slightly divergent and it encounters the rear converging optical unit which is arranged to form an image at a location that is accessible for observation.
The optical elements constituting the optical relay preferably all have spherical faces.
Each of the converging optical units can comprise one or more lenses.
The correcting meniscus can be placed upstream or preferably downstream of the geometrical center of the optical relay, all parallel beams from the front optical unit converging on said geometrical center.
The center of curvature of the meniscus can coincide initially with the geometrical center of the optical relay, i.e. prior to subsequent adjustments.
By moving the center of curvature of the meniscus away from the geometrical center of the relay during adjustment, it is possible to correct off-axis aberrations since the above-mentioned inclined parallel beams are no longer centered on the meniscus, while the axial parallel beam remains unaffected.
It is thus possible to correct both coma and astigmatism to a very large extent without thereby affecting the initial axial adjustment of the optical elements of the relay.
A diverging lens can be placed in front of the rear converging optical unit, thus enabling various corrections to be improved on the axis and off-axis.
At least one of the optical elements of the relay, i.e. either the above-mentioned diverging lens or the meniscus or some other element of the optical relay is preferably formed by a doublet of two types of glass having substantially the same refractive index and Abbe numbers that are very different.
Thus, modifying the curvature of the internal face of the doublet makes it possible to correct chromatism to a very large extent without affecting the convergence of the doublet, and thus without affecting earlier geometrical adjustments.
In a particular embodiment, the optical system is arranged as a reflector telescope comprising a primary mirror and a secondary mirror, and having an aberration-correcting optical relay as defined above.
Under such circumstances, the optical relay is advantageously placed downstream from the assembly constituted by the primary and secondary mirrors such that an image point from said assembly coincides with the object point of the optical relay.
This makes it possible to provide overall correction of aberrations imparted by the primary and secondary mirrors.
The optical relay may have two lenses forming the front converging optical unit, one lens forming the meniscus, a diverging lens, and two lenses forming the rear converging optical unit, giving a total of six lenses.
The reflector telescope of the invention can be arranged in particular as a Newtonian type telescope comprising a primary mirror of spherical shape and a secondary mirror that is plane, i.e. mirrors that are of shapes that are simple and easy to make, while nevertheless making it possible to obtain images of good quality by virtue of the optical relay.
In a particular embodiment of a Newtonian type telescope of the invention, the diameter of the secondary mirror is much smaller than the diameter of the primary mirror.
The diameter of the secondary mirror is preferably about 10% to 15% the diameter of the primary mirror.
It is recalled that the diameter of the secondary mirror is about 25% of the diameter of the primary mirror in a conventional Newtonian type telescope.
Since the secondary mirror is placed on the path of the beam that is incident on the primary mirror, it is advantageous for the secondary mirror to be as small as possible so as to be collect a largest possible quantity of light and minimize the effect of diffraction induced by the edges of the secondary mirror.
The present invention thus causes the above drawbacks to disappear, at least in part, by using a secondary mirror that is much smaller in diameter than the primary mirror.
Such a reduction in the diameter of the secondary mirror is made possible by using the optical relay which enables the image from the secondary mirror to be transferred, which image can thus be formed close to the relay, at a location that is easier to access for observation.
In addition, because the image is transferred, it is possible to reduce the length of the central tube of the telescope.
Still in the context of a Newtonian type telescope, the optical relay preferably comprises a meniscus formed by a doublet of two types of glass as described above together with a diverging lens formed by a single type of glass placed in front of the rear converging optical unit.
In another particular embodiment, the reflector telescope of the invention is arranged as a Cassegrain type telescope and has a primary mirror of spherical shape and a secondary mirror that is also of spherical shape.
In a particular embodiment of the Cassegrain type telescope, the secondary mirror is much smaller in diameter than the primary mirror.
The diameter of the secondary mirror is preferably about 20% to 25% the diameter of the primary mirror.
It is recalled that the diameter of the secondary mirror is about 35% to 45% the diameter of the primary mirror with a conventional Cassegrain type telescope.
This reduction in the diameter of the secondary mirror is made possible by using the optical relay which makes it possible to bring the image formed by the secondary mirror much closer to the secondary mirror.
The other advantages mentioned above for a Newtonian type telescope are also observed with a Cassegrain type telescope.
With a Cassegrain type telescope, the optical relay preferably has a meniscus formed by a single type of glass and a diverging lens formed by a doublet of two types of glass having the same refractive index and Abbe numbers that are very different.
The secondary mirror of the Cassegrain type telescope of the invention is advantageously a Mangin type mirror, i.e. reflection takes place on the rear face of a diverging lens constituting the Mangin mirror.
This achieves almost perfect correction of spherical aberration.
In another particular embodiment, the reflector telescope of the invention is arranged as a Newtonian type telescope having a primary mirror of spherical shape oriented in such a manner as to direct the converging light beam towards a point that is situated close to the inside wall of the telescope where the secondary mirror is located.
Because of its offset position, close to the inside wall of the telescope, the secondary mirror constitutes no more than a minimal obstruction for the incident beam on the primary mirror.
In this particular embodiment, the secondary mirror is advantageously fixed to the inside wall of the telescope by a single fixing means.
This eliminates the diffraction effect imparted by the fixing for the secondary mirror.
Still in this particular embodiment, the optical relay may be inclined relative to the side wall of the telescope.
This enables the volume of the telescope to be reduced.
In a particular embodiment of a Cassegrain type telescope of the invention, the spherical primary mirror is oriented in such a manner as to direct the converging light beam towards a point situated close to the inside wall of the telescope where the secondary mirror is located.
Thus, because of its offset position, close to the inside wall of the telescope, the secondary mirror constitutes no more than a minimal obstruction for the light beam incident on the primary mirror.
Advantageously, the secondary mirror is fixed to the inside wall of the telescope by a single fixing means, thus eliminating the diffraction effect induced by the fixing of the secondary mirror.
The invention thus makes it possible to provide an optical system that is arranged as a binocular telescope by associating two Cassegrain type telescopes in the particular configuration described immediately above, each telescope having an eyepiece and both telescopes being disposed in parallel, the eyepieces being spaced apart from each other by a distance that corresponds to the spacing between the eyes of a user.